1. Field of the Invention
The present invention relates generally to an interface test system, and relates more specifically to a test block and test plug for relay and meter testing.
2. Description of Related Art
Test blocks, test plugs, test probes, and related devices are most often found in direct association with current transformers, potential transformers, metering apparatus, and protective relay systems. The supply of electricity must be continuous and uninterrupted by breakdown. In the early days of electricity, spectacular failures were commonplace. Accurate revenue metering devices, protective relays, and circuit interrupting devices were thus developed. These devices typically were complex electromechanical assemblies. Mechanical parts needed to be lubricated, adjusted, and aligned. Electrical parts eroded, burned, and failed in use. The electrical testing and maintenance industry is born.
Alternating current becomes dominant with electric power conductors carrying larger and larger currents. Known metering and relay devices were no longer able to accommodate these currents directly. For high current, AC, power circuits, the current transformer was developed and used to provide a proportional, accurate current interface. Meter and relay current input standardization followed. However, the current transformer solution is limited. The secondary current loop must not be switched opened or disconnected when the primary side, power circuit is energized. In such a configuration, the current transformer fails catastrophically as a result of over-excitation, magnetic core saturation, over temperature, core interleaf, and winding insulation breakdown.
It is probably that in response to the disadvantages of the current transformer, the test block was designed. A first early example is shown in FIG. 1, and is a control rod, being a General Electric test block device. The operator's current meter was made active when the control rod was inserted into the board-mounted receptacle. Insertion of the rod separated the two parts of a switch. The switch was connected across the current transformer secondary terminals and in parallel with the current meter terminals. With the rod removed, the switch is closed; current flow is from the transformer through the switch and back to the transformer completing the secondary loop. In this condition, the current meter could then be safely removed for calibration. The control rod is made from an insulating material. The rod is fitted with a brass tip that provides a make before break switch action.
Developed in the 1930s, the General Electric, Type PK-2 test block (with keyed cover) is the genesis of all testing devices. Generally, the PK-2 was cased in a Bakelite casing housing ten contacts that are closed when a matching plug is inserted. Thus, the plug must be inserted at all times and will only be removed when opening the circuits. Short circuits are achieved inside the test block, but this time by removal of an item. This system has been rendered unsafe for the use in nuclear power stations since the 1970s but has been in use in various places up to today nevertheless.
The example shown in FIG. 2 is a device including eight terminals configured as four, entry side and four, exit side. The connected instrument, relay or meter, is made active when the cover plug is inserted into the block. Four copper conductor bridges internal to the cover complete the connection between entry and exit. When the cover is removed, a shunt in the block side is switched across two of the entry side terminals that would be used for the current transformer secondary. With the cover removed, the associated relay or meter is isolated and could be disconnected for service or replacement.
As the test block evolved, the Westinghouse Type FT (Flexitest) was developed, and was a test block with an insulated, draw-out style, protective relay case. Such a device is described in a Westinghouse Instruction Manual, I.L.41-076D, effective February 1965. The test block at the lower edge of the case contains two color, coded “knife-blade test switches.” Westinghouse recommends: “Always open the red handle switches first before any of the black handle switches . . . ” The individual knife-blade test switches provide the test technician with individual switching control of each external connection. The reference to the red handles is a recommendation to open the trip circuits first to prevent an accidental trip out of the circuit breaker. Some of the knife-blade switches are configured for current transformer connection. When opened, the current transformer configured knife-blade switch causes the engagement of a shorting jaw across the current transformer secondary (while isolating the relay current input for test purposes.) Opening all test switches isolates the relay. Next a multi-circuit test plug is inserted that permits external connection to the relay for relay testing “in case.” Or, alternately the relay can be removed from the case for replacement or testing “out of case.” The Type FT (Flexitest) case with accessory test plugs and probes, provides three relay test modes described by the instructions as follows: Testing in Service, Testing In Case, and Testing Out of Case.
The test block is still currently in use today. The device continues to evolve and adapt. Digital relays have not eliminated the need for the test block (as predicted by some). In response to industry needs, test block development and innovation continues to drive new products to the market.
Although test block systems are known, FT switches represent the most common comparative usage prevalent in the US. They are manufactured in various sizes by various manufacturers—the most prominent being ABB. The FT switch includes two stainless steel parts, one of which is equipped with an angled connector that can be pressed into a fitting indenture of the other part, thus making connection. To open the circuit, the angling part has to be removed out of the indenture manually. For this usage, the angling part is equipped with a plastic cover. Theoretically, FT switches are also modular in usage, but they are usually housed in a plastic casing and come in groups of ten. ABB provides test handles for ten contacts, which can be inserted after all ten contacts have been opened individually. These test handles also provide stainless steel as conductive material.
FT switches incorporate disadvantages, including that the contacts have to be opened individually, the user needs to know the whole setup of the system; manual opening and closing makes way for human error that can be dangerous; manual opening puts the user at risk of electrical shock; the used material (stud-welded stainless steel) has high internal resistance of 200-300 Milliohms and reaches temperatures of more than 70 degrees Celsius; and all operations (elimination of CTs and PTs, qualified injection) have to be performed by highly trained professionals.
Test blocks, test plugs, and probes are available today with decidedly different functionality and technical characteristics. This range is best understood by the comparison of products offered by two manufacturers, ABB and AREVA, in view of the present invention.
Product development by each manufacturer reveals a concentration of test characteristics either in the block, or the plug. The test block is the electrical switching device that is permanently wired into the operating circuits, typically in the face or back panel of a substation cabinet, or 19-inch rack. A focus on the test block is found in the ABB products (Westinghouse legacy) and in the AREVA products. A disadvantage of the block focus is that it places the test characteristics (higher cost and complexity) in the device installed in the substation panel.
An example of one ABB test block product is the ABB FT-1 Switch, which is constructed with a one-piece, molded polycarbonate base unit that can be configured with between 1 and 10 switch poles. Three FT-1 Switch units can be grouped together in the FT-19R, 19-inch rack assembly (30 poles). In the FT-1, individual switch poles are generally either potential poles or current poles. Potential poles are non-shorting knife blades (potential, trip, or control.) Current poles are configured in sets of two; the configuration includes a current test jack, a shorting spring, a shorting blade, and a non-shorting blade. Current pole handles are typically color coded black. Potential pole handles are typically red or black. Other handle color options include: green, yellow, blue, white, and orange. This ABB unit offers only manual operation.
ABB offers three test plug versions for use with the FT-1 Switch: (1) in-service series, (2) individual current circuit test plug, and (3) separate source test plug. The “in-service series” test plug, with up to ten poles, is used to make a connection between external test instruments and active currents and voltages (with the relay or other switchboard device in service.) The “individual current circuit test plug” is the current probe option that may be used to connect an external meter into an active current loop. The ABB knife blade switch must be in the open position to permit insertion of the current probe. The third plug option, “separate source test plug” is used to isolate and connect the “out of service” relay or switchboard device to external test instruments. The “separate source test plug” is inserted after all ten poles of the FT-1 Switch (test block) are open.
An example of one AREVA test block product is the AREVA P990 Test Block, which is designed to contain 14 cassettes. Each cassette contains a switch contact pair. The cassettes are available in five types: Stage I, Stage II, Stage III, CT, and Blank. The three stage cassettes provide the three steps of an automatic operating sequence. The CT cassette includes provision for shorting the current transformer loop. This AREVA unit offers an automatic operating sequence.
The AREVA P992 multi-finger test plug is used with the P990 test block to connect external test instruments to the relay (or other switchboard device.) The insertion of the P992 test plug opens circuits in a pre-configured sequence of up to three steps. The insertion of the plug safely shorts current transformer circuits. The AREVA P993 single-finger test plug is used to measure an individual current circuit.
The AREVA P992 and P993 probe finger design causes a momentary opposite polarity circuit connection during insertion and extraction. See FIG. 3. This momentary polarity reversal is a serious design flaw that may cause damage to any connected device with a grounded (earthed) power supply input terminal. Refer to the AREVA instructions for more information about this hazard.
Therefore, it can be seen that a need yet exists for a superior interface test system. It is to such a system that the present invention is primarily directed.